An MRI scan showing a penis in a vagina is still popular with readers 20 years later, doctors have revealed.
The images of heterosexual couples in various sex positions were part of a Dutch study published in the British Medical Journal (BMJ) in 1999, the Daily Mail reported.
One image in particular shows the couple in missionary position with the penis taking on a curved shape - looking similar to a boomerang. Before the study, it was always presumed that the sex organ was either straight or 'S'-shaped when entering the vagina.
At the time of publishing their paper, led by the University Hospital Groningen and VU Amsterdam University, nobody at BMJ thought it would be of any importance. Boy, were they wrong. It went on to become one of the journal's most downloaded articles of all time and has been cited in 130 other scientific papers since.
One image in particular shows the couple in missionary position with the penis taking on a curved shape - looking similar to a boomerang. Picture: BMJ.com
Now, nearly two decades after its findings, it's been accessed more than 50 times in October this year alone. And now experts want to know why.
A former editor of the BMJ said: "It was hardly the medical equivalent of a moon landing, so why did 'lay' visitors come flocking in such numbers?"
The study's authors think they have some inkling as to why. They believe the prospect of seeing sex on screen, for free, was behind its success - even if they were a series of black and white still images.
The novelty continues to live on as the photos get passed down from the previous generation.
According to various reports, hospital officials were originally reluctant to go ahead with the study and the researchers ran into problems with sexual performance - although they never specified what the issues were.
They eventually captured the first magnetic resonance imaging of sex in progress almost 20 years after the technology became popular.